Signal processing for a capacitive sensor system with robustness to noise

ABSTRACT

A capacitive sensor includes a transmit electrode configured to provide an alternating electric field to a sensor; one or more receive electrodes for detecting variations in the alternating electric field; and an adaptive frequency adjustment unit configured to adjust an operating frequency of the alternating electric field responsive to detection of a noise measure, such as noise power.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 61/684,009, filed Aug. 16, 2012, which is herebyincorporated by reference in its entirety as if fully set forth herein.

TECHNICAL FIELD

The present disclosure relates to methods and systems for capacitivesensor systems, in particular to signal processing in such systems.

BACKGROUND

Capacitive sensor systems can be realized by generating an alternatingelectrical field and measuring the potential difference (i.e., thevoltage) obtained in one cycle at a sensor electrode within this field.A single electrode or a transmitting and one or more receivingelectrodes may be used. This voltage is a measure for the capacitancebetween the sensor electrode and its electrical environment, i.e., it isinfluenced by objects like a human finger or a hand. Further, from thisvoltage, for example, the distance of a finger can be deduced. Thisinformation can be used for human-machine interfaces.

The problem with conventional systems operating according to theabove-mentioned principle is that electrical noise sources, such asfluorescent lamps or USB chargers can affect the electrical field. Thus,accurately and reliably estimating this voltage in a noisy environmentis problematic.

SUMMARY

According to various embodiments, a capacitive sensor system is providedthat implements automatic adaptation of an operating frequency inenvironments with frequency selective noise. A capacitive sensoraccording to embodiments includes a transmit electrode configured toprovide an alternating electric field to a sensor; one or more receiveelectrodes for detecting variations in the alternating electric field;and an adaptive frequency adjustment unit configured to adjust anoperating frequency of the alternating electric field responsive todetection of a noise measure, such as noise power. In some embodiments,the adaptive frequency adjustment unit is configured to determine aplurality of noise powers at potential operating frequencies and selecta new operating frequency.

A method for providing noise robustness to a capacitive sensing systemaccording to embodiments includes defining a plurality of potentialoperating transmit frequencies; determining a corresponding noisemeasure corresponding to each of the potential operating frequencies;operating the capacitive sensing system at one of the plurality ofpotential operating transmit frequencies; measuring an operating noisemeasure at the operating frequency; and selecting a new operatingfrequency responsive to a value of the measured operating noise measure.

A capacitive sensor system according to embodiments includes a transmitelectrode configured to provide an alternating electric field to asensor; one or more receive electrodes for detecting variations in thealternating electric field; and an adaptive frequency adjustment unitconfigured to adjust an operating frequency of the alternating electricfield in accordance with modeling the capacitive sensor system as anamplitude modulation system with direct sampling and synchronousdemodulation. In some embodiments, the adaptive frequency adjustmentunit is configured to determine a plurality of noise measures atpotential operating frequencies and select an operating frequency thatcorresponds to a minimum or maximum of the plurality of noise measures.

A sensor system according to embodiments includes an alternatingelectric field sensor arrangement being subject to noise and beingcoupled with a signal processing unit receiving a noisy signal, whereinthe signal processing unit converts the noisy signal to a digitalsignal, wherein the signal processing unit is further configured todemodulate the sampled signal by multiplication with (−1)^(k), where kindicates the discrete time, subsequent low-pass filtering, subsequentdecimation by a factor R, and further low-pass filtering. The signalprocessing unit may be further operable to perform a distanceestimation, positioning or gesture recognition from the processedsignal. The alternating electric field may be generated by a pulsesignal.

These, and other, aspects of the disclosure will be better appreciatedand understood when considered in conjunction with the followingdescription and the accompanying drawings. It should be understood,however, that the following description, while indicating variousembodiments of the disclosure and numerous specific details thereof, isgiven by way of illustration and not of limitation. Many substitutions,modifications, additions and/or rearrangements may be made within thescope of the disclosure without departing from the spirit thereof, andthe disclosure includes all such substitutions, modifications, additionsand/or rearrangements.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings accompanying and forming part of this specification areincluded to depict certain aspects of the disclosure. It should be notedthat the features illustrated in the drawings are not necessarily drawnto scale. A more complete understanding of the disclosure and theadvantages thereof may be acquired by referring to the followingdescription, taken in conjunction with the accompanying drawings inwhich like reference numbers indicate like features and wherein:

FIG. 1 depicts a diagrammatic representation of exemplary capacitivesensing.

FIG. 2 depicts a diagrammatic representation of an exemplary capacitivesensor.

FIG. 3 illustrates exemplary noise and noise abatement.

FIG. 4 depicts a diagrammatic representation of an exemplary capacitivesensor.

FIG. 5 depicts the exemplary capacitive sensor of FIG. 4 in greaterdetail.

FIG. 6 is an exemplary noise power spectral density.

FIG. 7 is an exemplary power spectral density of a noisy input signal.

FIG. 8 is an exemplary power spectral density of a sampled, noisy inputsignal.

FIG. 9 is an schematic spectrum showing signals of interest.

FIG. 10 is a diagram illustrating process flow according to embodiments.

FIG. 11 is an exemplary system implementing methods according toembodiments.

DETAILED DESCRIPTION

The disclosure and various features and advantageous details thereof areexplained more fully with reference to the exemplary, and thereforenon-limiting, embodiments illustrated in the accompanying drawings anddetailed in the following description. It should be understood, however,that the detailed description and the specific examples, whileindicating the preferred embodiments, are given by way of illustrationonly and not by way of limitation. Descriptions of known programmingtechniques, computer software, hardware, operating platforms andprotocols may be omitted so as not to unnecessarily obscure thedisclosure in detail. Various substitutions, modifications, additionsand/or rearrangements within the spirit and/or scope of the underlyinginventive concept will become apparent to those skilled in the art fromthis disclosure.

Turning now to the drawings and with particular attention to FIG. 1, anexemplary sensor electrode arrangement 100 for evaluation of analternating electric field is shown. The sensor electrode arrangementincludes a plurality of receive electrodes 106 a-106 e and one or moretransmit electrodes 104. The one or more receive electrodes 106 a-e aretypically arranged in a layer above the transmit electrode 104, and withan insulating layer (not shown) arranged between the transmit andreceive electrodes. The sensor electrode arrangement 100 is configuredto determine an effect that an object, such as finger 102, has on thealternating electric field. A distance 108 between the finger 102 andthe sensor electrode arrangement may thereby be determined.

More particularly, according to various embodiments, a front end deviceestimates the distance between finger 102 and the sensor arrangement bymeasuring the capacitance between the receive electrodes and the finger(GND). Potential changes at a capacitive voltage divider are excited byan alternating voltage.

This is illustrated schematically with reference to FIG. 2. Inparticular, FIG. 2 shows a human finger 102 at distance x_(O) over thecross-section of a capacitive sensor system 200 with a transmitterelectrode E_(TX) and a receiver electrode E_(RX). E_(TX) is excitedusing a rectangular pulse train voltage source 202, where the sourceresistance R_(s) and the capacitance C_(TX) between E_(TX) and ground(GND) form a low-pass filter. The rectangular pulse train voltagetypically has a frequency of 40-140 kHz. The variable capacitance C_(f)between E_(RX) and GND is dominated by the capacitance between E_(RX)and the finger 102, which depends on the distance x_(O) between fingerand E_(RX). C_(f) and the constant capacitance C_(s) between E_(TX) andE_(RX) build a capacitive voltage divider 204. Hence, the voltage V_(f)is a function of x_(O).

It is noted that further capacitances which are of minor importance forthe basic understanding of the sensor system are omitted in the drawing.Also, the capacitance C_(f) not only depends on the distance x_(O), buton the three dimensional position of the finger tip, orientation of thehand, hand size, etc.

FIG. 3 illustrates output signals 300, 302 of an integrated front enddevice. Output signal 300 shows the output signal without noiseremediation, i.e., automatic frequency adaptation, according toembodiments, while output signal 302 is the output with automaticfrequency adaptation according to embodiments. As shown, a noise source,such as a fluorescent lamp, is switched on at 304. The sensor signal 307a shows the effect of the noise, while the signal 307 b shows a clean,noise-free signal.

As will be explained in greater detail below, according to variousembodiments, noise robustness to the sensor electrode arrangement can beprovided, for example, to an integrated front end circuit for directevaluation of the sensor signals. According to various embodiments,channel noise can be automatically evaluated and best operatingfrequencies can be selected.

FIG. 4 illustrates an exemplary capacitive sensor 400 according tovarious embodiments. The output of the voltage source V_(TX) is lowpassfiltered at 403 and drives a capacitive voltage divider 404. The receiveelectrode E_(RX) may be connected to an optional buffer 406, an analogband-pass filter 408, and an analog-to-digital converter (ADC) 410,which performs direct sampling. The ADC 410 may be synchronized with thevoltage source to take two samples each transmitter period. The ADC'soutput is fed into a digital signal processing (DSP) unit 412. As willbe explained in greater detail below, the DSP 412 controls theoscillator V_(TX) via control path 414 to select an optimal operationalfrequency in the face of noisy conditions.

FIG. 5 illustrates a communication theoretic model of the capacitivesensor of FIG. 4. In particular, as will be explained in greater detailbelow, the capacitive sensor may be modeled as an amplitude modulationsystem with direct sampling. An exemplary sampling frequency may be, forexample, twice the frequency of the transmit signal V_(TX). For example,if f_(TX) is 100 kHz, then f_(s)=1/T_(s)=2f_(TX)=200 kHz. It is notedthat other frequencies are possible.

The digital signal processing block 412 may implement digitaldemodulation 506, downsampling by R (e.g., to f_(s)′=200 Hz withR=1000), i.e. low-pass filtering 508 with subsequent decimation 510, lowpass filtering 512, frequency dependent signal adjustment 514, andsubsequent positioning and gesture recognition 516 according to variousembodiments.

More particularly, the output of the lowpass filter 403, i.e., thelowpass filtered rectangular pulse train 402 may be modeled as a carriersignal c(t), where t denotes continuous time. The carrier signal c(t) ismodulated 501 with [m0+m(t)], which is a function of the capacitanceC_(f) (FIG. 2, FIG. 4), yielding y(t).

At 502, the signal y(t) has added to it random noise e(t). The randomnoise e(t) may be representative of, for example, noise from fluorescentlight bulbs or other sources. The resulting noisy signal z(t)=y(t)+e(t)is sampled 504 at discrete times k*T_(s), where T_(s)=1/(2*f_(TX)) isthe inverse of twice the transmitter frequency f_(TX) and k=0, 1, 2, . .. is the discrete time index.

The ADC 410 converts the time-discrete signal into the digital domain.The ADC output z(k) is then processed by the DSP 412. In the exampleillustrated, the signal z(k) is digitally demodulated at 506 bymultiplication with (−1)^(k), low-pass filtered 508, decimated by factorR (typically 400-1400) 510, and low-pass filtered 512 a second time toonly contain frequencies of hand movement (typically 0-20 Hz). It isthen used for further processing like distance estimation, positioningor gesture recognition 516. An automatic frequency adaptation (AFA)module 514 receives the decimator output and provides control signalsfor adjusting the signal to provide robustness to frequency selectivenoise.

FIG. 6 shows the Power Spectral Density (PSD) of a fluorescent lamp asan example of an external noise source. This PSD, which has beenmeasured at the ADC input of the system, shows harmonic narrow-bandemission. If the system's TX frequency f_(TX) (i.e., the carrierfrequency) meets one of these emissions, the system is affected inrecognizing a user's input. FIG. 7 shows a more detailed spectrum of anexternal noise source like a fluorescent lamp. It also includes thedistinct peaks of the system's rectangular TX signal with an exemplaryfrequency of 70 kHz. In this example, f_(TX) and its harmonics do notcoincide with the emissions of the noise source.

Depending on the steepness of the analog band-pass filter 408, evenemissions at multiples of f_(TX) will affect the system. For example,FIG. 8 shows the 3rd harmonic of the 47 kHz noise peak in the PSD inFIG. 6 folded into the 200 kHz band at (200−3*47)=59 kHz due to samplingafter an insufficiently steep analog band-pass filter.

The spectrum obtained after down-sampling 510 is depicted schematicallyin FIG. 9. The spectrum of the down-sampled signal is narrow compared tonoise peaks in FIG. 8. External noise is now flat in the PSD. Theestimate of external noise power 908 lies in a frequency band of, forexample, 70-90 Hz.

The components of the spectrum after downsampling are a) the wantedsignal (0-20 Hz) 902, b) known low-frequency noise, e.g., mains (i.e.,50 or 60 Hz line) voltage 904, which has been modulated onto the carrierdue to non-linear system components, and c) a noise floor 906 that isrepresentative of high frequency sources such as a fluorescent light.The noise floor 906 is approximately flat at the frequencies of interestand is relatively low if no high-frequency (HF) noise is present.

This noise floor 906 will rise if the current f_(TX) lies in a noisyfrequency band, whose width exceeds the sampling rate of the demodulatedsignal (which is the typical case). Hence, the HF noise power can bemeasured in the down-sampled signal in any frequency band that does notcontain the wanted signal 902 or a known low-frequency noise 904, suchas in band 908.

According to various embodiments, the Automatic Frequency Adaptation(AFA) unit 518 employs this noise power measurement technique. Othernoise measures are possible, however.

Turning now to FIG. 10, a diagram illustrating an exemplary processaccording to embodiments is shown. At 1002, the AFA 518 performs aseries of measurements of noise power on a pre-defined set 1004 of TXfrequencies (e.g., eight frequencies in the range 40-140 kHz) atsystem's start-up or if no user activity is recognized (or on any othersuitable event). Measurements are made, for example, in a band such as908, shown in FIG. 9. The respective measured noise powers are saved ina dedicated array 1006. On an exit criterion, e.g., if all frequenciesin the set 1004 have been investigated or the start of a user's activityis recognized, the system operates at a fixed TX frequency (f_(TX)).This frequency may be chosen to be the one that shows the lowest valuein the array of noise powers 1006.

While the user is active, the AFA periodically measures the noise power1008 at the current operating frequency f_(TX). If the value of thenoise power exceeds a certain threshold, the TX frequency f is adjustedto a different frequency having a lower noise power. For example, insome embodiments, the frequency is changed to the frequency thatcorresponds to the lowest value in the array of noise powers 1006.

Additionally, in some embodiments, the AFA may trigger frequencydependent adjustments in the DSP, e.g., the rate of down-sampling, andcan make adjustments to provide subsequent processing steps withfrequency independent data. The information on signal reliability ispassed to those DSP blocks, too. There, e.g., filter gains are adjusteddepending on signal reliability. Also adjusted can be signal levelthresholds of probability thresholds for triggering some functionality,or other, e.g. probabilistic parameters.

If the measured noise values exceed pre-defined thresholds at allfrequencies, the system can be interpreted as inoperable, which can besignaled to the host unit (FIG. 11). Then, the search for a noise-freeoperating frequency is continued.

In another embodiment, noise detection may be implemented by computingthe ratio of the power of low-pass filtered signal 902 and over-allpower and to comparing this to a pre-defined threshold. Alternatively,any sub-band not containing hand gestures can be evaluated.

Extensive investigation showed that the spectral noise bands offluorescent lamps and USB chargers typically are significantly broaderthan the decimated sampling rate, which justifies the assumption of arelatively flat noise spectrum in the decimated signal. This allows fordetection of HF noise sources in a sub-band of the decimated signal,which is not covered by the target signal or other known low-frequencynoise sources.

Turning now to FIG. 11, a block diagram illustrating a particularimplementation of a sensor system 1100 including noise robustnessaccording to embodiments. The system 1100 includes a sensing controller1101, sensing electrodes 1102, and a host system 1103. The sensingelectrodes 1102 may implement a configuration such as shown in FIG. 1.The host 1103 may be any system that can make use of capacitive sensorsignals and/or information or data derived therefrom, such as cellphones, laptop computers, I/O devices, and the like.

In the example illustrated, a TX signal generator 1104 provides thetransmitter signal V_(TX) to the transmit electrode TXD. Receiveelectrodes RX0-RX4 are received at signal conditioning modules 1106 forimplementing filtering, etc. The outputs of signal conditioning areprovided to ADCs 1107 and, via signal lines or other medium such as abus 1108, to a signal processing unit 1108. The signal processing unit1108 may implement the functionality of the DSP (FIG. 5, FIG. 10).Resulting outputs may be provided via IO unit 1118 to the host 1103.

The system may further include a variety of additional modules, such asinternal clock 1109, memory such as flash memory 1112, a voltagereference 1110, power management 1114, low-power wake-up 1116, resetcontrol 1122, and communication control 1120.

Although the invention has been described with respect to specificembodiments thereof, these embodiments are merely illustrative, and notrestrictive of the invention. The description herein of illustratedembodiments of the invention, including the description in the Abstractand Summary, is not intended to be exhaustive or to limit the inventionto the precise forms disclosed herein (and in particular, the inclusionof any particular embodiment, feature or function within the Abstract orSummary is not intended to limit the scope of the invention to suchembodiment, feature or function). Rather, the description is intended todescribe illustrative embodiments, features and functions in order toprovide a person of ordinary skill in the art context to understand theinvention without limiting the invention to any particularly describedembodiment, feature or function, including any such embodiment featureor function described in the Abstract or Summary. While specificembodiments of, and examples for, the invention are described herein forillustrative purposes only, various equivalent modifications arepossible within the spirit and scope of the invention, as those skilledin the relevant art will recognize and appreciate. As indicated, thesemodifications may be made to the invention in light of the foregoingdescription of illustrated embodiments of the invention and are to beincluded within the spirit and scope of the invention. Thus, while theinvention has been described herein with reference to particularembodiments thereof, a latitude of modification, various changes andsubstitutions are intended in the foregoing disclosures, and it will beappreciated that in some instances some features of embodiments of theinvention will be employed without a corresponding use of other featureswithout departing from the scope and spirit of the invention as setforth. Therefore, many modifications may be made to adapt a particularsituation or material to the essential scope and spirit of theinvention.

Reference throughout this specification to “one embodiment”, “anembodiment”, or “a specific embodiment” or similar terminology meansthat a particular feature, structure, or characteristic described inconnection with the embodiment is included in at least one embodimentand may not necessarily be present in all embodiments. Thus, respectiveappearances of the phrases “in one embodiment”, “in an embodiment”, or“in a specific embodiment” or similar terminology in various placesthroughout this specification are not necessarily referring to the sameembodiment. Furthermore, the particular features, structures, orcharacteristics of any particular embodiment may be combined in anysuitable manner with one or more other embodiments. It is to beunderstood that other variations and modifications of the embodimentsdescribed and illustrated herein are possible in light of the teachingsherein and are to be considered as part of the spirit and scope of theinvention.

In the description herein, numerous specific details are provided, suchas examples of components and/or methods, to provide a thoroughunderstanding of embodiments of the invention. One skilled in therelevant art will recognize, however, that an embodiment may be able tobe practiced without one or more of the specific details, or with otherapparatus, systems, assemblies, methods, components, materials, parts,and/or the like. In other instances, well-known structures, components,systems, materials, or operations are not specifically shown ordescribed in detail to avoid obscuring aspects of embodiments of theinvention. While the invention may be illustrated by using a particularembodiment, this is not and does not limit the invention to anyparticular embodiment and a person of ordinary skill in the art willrecognize that additional embodiments are readily understandable and area part of this invention.

Any suitable programming language can be used to implement the routines,methods or programs of embodiments of the invention described herein,including C, C++, Java, assembly language, etc. Different programmingtechniques can be employed such as procedural or object oriented. Anyparticular routine can execute on a single computer processing device ormultiple computer processing devices, a single computer processor ormultiple computer processors. Data may be stored in a single storagemedium or distributed through multiple storage mediums, and may residein a single database or multiple databases (or other data storagetechniques). Although the steps, operations, or computations may bepresented in a specific order, this order may be changed in differentembodiments. In some embodiments, to the extent multiple steps are shownas sequential in this specification, some combination of such steps inalternative embodiments may be performed at the same time. The sequenceof operations described herein can be interrupted, suspended, orotherwise controlled by another process, such as an operating system,kernel, etc. The routines can operate in an operating system environmentor as stand-alone routines. Functions, routines, methods, steps andoperations described herein can be performed in hardware, software,firmware or any combination thereof.

Embodiments described herein can be implemented in the form of controllogic in software or hardware or a combination of both. The controllogic may be stored in an information storage medium, such as acomputer-readable medium, as a plurality of instructions adapted todirect an information processing device to perform a set of stepsdisclosed in the various embodiments. Based on the disclosure andteachings provided herein, a person of ordinary skill in the art willappreciate other ways and/or methods to implement the invention.

It is also within the spirit and scope of the invention to implement insoftware programming or code any of the steps, operations, methods,routines or portions thereof described herein, where such softwareprogramming or code can be stored in a computer-readable medium and canbe operated on by a processor to permit a computer to perform any of thesteps, operations, methods, routines or portions thereof describedherein. The invention may be implemented by using software programmingor code in one or more general purpose digital computers, by usingapplication specific integrated circuits, programmable logic devices,field programmable gate arrays, and so on. Optical, chemical,biological, quantum or nanoengineered systems, components and mechanismsmay be used. In general, the functions of the invention can be achievedby any means as is known in the art. For example, distributed, ornetworked systems, components and circuits can be used. In anotherexample, communication or transfer (or otherwise moving from one placeto another) of data may be wired, wireless, or by any other means.

A “computer-readable medium” may be any medium that can contain, store,communicate, propagate, or transport the program for use by or inconnection with the instruction execution system, apparatus, system ordevice. The computer readable medium can be, by way of example only butnot by limitation, an electronic, magnetic, optical, electromagnetic,infrared, or semiconductor system, apparatus, system, device,propagation medium, or computer memory. Such computer-readable mediumshall generally be machine readable and include software programming orcode that can be human readable (e.g., source code) or machine readable(e.g., object code). Examples of non-transitory computer-readable mediacan include random access memories, read-only memories, hard drives,data cartridges, magnetic tapes, floppy diskettes, flash memory drives,optical data storage devices, compact-disc read-only memories, and otherappropriate computer memories and data storage devices. In anillustrative embodiment, some or all of the software components mayreside on a single server computer or on any combination of separateserver computers. As one skilled in the art can appreciate, a computerprogram product implementing an embodiment disclosed herein may compriseone or more non-transitory computer readable media storing computerinstructions translatable by one or more processors in a computingenvironment.

A “processor” includes any, hardware system, mechanism or component thatprocesses data, signals or other information. A processor can include asystem with a general-purpose central processing unit, multipleprocessing units, dedicated circuitry for achieving functionality, orother systems. Processing need not be limited to a geographic location,or have temporal limitations. For example, a processor can perform itsfunctions in “real-time,” “offline,” in a “batch mode,” etc. Portions ofprocessing can be performed at different times and at differentlocations, by different (or the same) processing systems.

As used herein, the terms “comprises,” “comprising,” “includes,”“including,” “has,” “having,” or any other variation thereof, areintended to cover a non-exclusive inclusion. For example, a process,product, article, or apparatus that comprises a list of elements is notnecessarily limited only those elements but may include other elementsnot expressly listed or inherent to such process, process, article, orapparatus.

Furthermore, the term “or” as used herein is generally intended to mean“and/or” unless otherwise indicated. For example, a condition A or B issatisfied by any one of the following: A is true (or present) and B isfalse (or not present), A is false (or not present) and B is true (orpresent), and both A and B are true (or present). As used herein,including the claims that follow, a term preceded by “a” or “an” (and“the” when antecedent basis is “a” or “an”) includes both singular andplural of such term, unless clearly indicated within the claim otherwise(i.e., that the reference “a” or “an” clearly indicates only thesingular or only the plural). Also, as used in the description hereinand throughout the claims that follow, the meaning of “in” includes “in”and “on” unless the context clearly dictates otherwise.

It will be appreciated that one or more of the elements depicted in thedrawings/figures can also be implemented in a more separated orintegrated manner, or even removed or rendered as inoperable in certaincases, as is useful in accordance with a particular application.Additionally, any signal arrows in the drawings/Figures should beconsidered only as exemplary, and not limiting, unless otherwisespecifically noted.

What is claimed is:
 1. A capacitive sensor, comprising: a transmitelectrode configured to provide an alternating electric field for thecapacitive sensor; an oscillator for generating a carrier signal; one ormore receive electrodes for detecting variations in the alternatingelectric field; an adaptive frequency adjustment unit coupled with theoscillator and configured to adjust an operating frequency of thealternating electric field responsive to detection of a noise measure,and a modulator configured to modulate the carrier signal at theoperating frequency depending on a sensed capacitance, wherein themodulated carrier signal is synchronously demodulated and downsampled.2. The capacitive sensor of claim 1, wherein the adaptive frequencyadjustment unit is configured to determine a plurality of noise measuresat potential operating frequencies and select a new operating frequency.3. The capacitive sensor of claim 2, wherein the new operating frequencycorresponds to a minimum or maximum of the plurality of noise measures.4. The capacitive sensor of claim 2, wherein the new operating frequencycorresponds to a sufficiently high or sufficiently low noise measure ofthe plurality of noise measures.
 5. The capacitive sensor of claim 2,wherein the noise measures comprise noise powers.
 6. The capacitivesensor of claim 2, wherein the noise measure is a measure for the noiseat the operating frequency.
 7. The capacitive sensor of claim 6, whereinthe noise measure is a measure for the noise power at the operatingfrequency.
 8. The capacitive sensor of claim 7, wherein a carrier signalat the operating frequency is modulated by a low-frequency target signalusing amplitude modulation.
 9. The capacitive sensor of claim 1, furthercomprising an analog-to-digital converter configured to convert ananalog signal received from the one or more receive electrodes into adigital signal, wherein a sampling frequency is twice the frequency ofthe carrier signal.
 10. The capacitive sensor of claim 8, wherein thenoise measure is a measure of the signal energy of the downsampledsignal in a frequency band that is unequal the frequency band of thelow-frequency modulating signal and unequal frequency bands that containlow-frequency noise.
 11. The capacitive sensor of claim 2, wherein theadaptive frequency adjustment unit is configured to select a newoperating frequency if a detected noise measure exceeds or falls below apredetermined threshold.
 12. The capacitive sensor of claim 11, whereinthe new operating frequency is an operating frequency that correspondsto a minimum or maximum of the plurality of noise measures.
 13. A methodfor providing noise robustness to a capacitive sensing system,comprising: generating a carrier signal and modulating the carriersignal at the operating frequency depending on a sensed capacitance;defining a plurality of potential operating transmit frequencies;determining a corresponding noise measure corresponding to each of thepotential operating frequencies; operating the capacitive sensing systemat one of the plurality of potential operating transmit frequencies;measuring an operating noise measure at the operating frequency; andselecting a new operating frequency responsive to a value of themeasured operating noise measure; wherein the modulated carrier signalis synchronously demodulated and downsampled.
 14. The method of claim13, wherein the new operating frequency corresponds to a minimum ormaximum of the plurality of noise measures.
 15. The method of claim 13,wherein the new operating frequency corresponds to a sufficiently highor sufficiently low noise measure of the plurality of noise measures.16. The method of claim 13, wherein the noise measures comprise noisepowers.
 17. The method of claim 13, wherein the noise measure is ameasure for the noise at the operating frequency.
 18. The method ofclaim 13, wherein the noise measure is a measure for the noise power atthe operating frequency.
 19. The method of claim 13, wherein a carriersignal at the operating frequency is modulated by a low-frequency targetsignal using amplitude modulation.
 20. The method of claim 13, furtherconverting an analog signal received from the one or more receiveelectrodes into a digital signal with a sampling frequency that is twicethe frequency of the carrier signal.
 21. The method of claim 19, whereinthe noise measure is a measure of the signal energy of the downsampledsignal in a frequency band that is unequal the frequency band of thelow-frequency modulating signal and unequal frequency bands that containlow-frequency noise.
 22. The method of claim 13, including selecting anew operating frequency if a detected noise measure exceeds or fallsbelow a predetermined threshold.
 23. The method of claim 22, wherein thenew operating frequency is an operating frequency that corresponds to aminimum or maximum of the plurality of noise measures.
 24. A capacitivesensor system, comprising: a transmit electrode configured to provide analternating electric field to a capacitive sensor; a plurality ofreception electrodes associated with the transmit electrode fordetecting variations in the alternating electric field generated by thetransmit electrode; and an adaptive frequency adjustment unit configuredto adjust an operating frequency of the alternating electric field inaccordance with modeling the capacitive sensor system as an amplitudemodulation system with direct sampling and synchronous demodulation. 25.The capacitive sensor system of claim 24, wherein the adaptive frequencyadjustment unit is configured to determine a plurality of noise measuresat potential operating frequencies and select an operating frequencythat corresponds to a minimum or maximum of the plurality of noisemeasures.
 26. The capacitive sensor system of claim 25, wherein theadaptive frequency adjustment unit is configured to select a newoperating frequency if a detected noise measure exceeds a predeterminedthreshold.
 27. The capacitive sensor system of claim 26, wherein thenoise measure is a noise power.
 28. The capacitive sensor system ofclaim 24, further comprising: an oscillator for generating a carriersignal coupled with the frequency adjustment unit; a modulator operableto modulate the carrier signal at the operating frequency depending on asensed capacitance, wherein the modulated carrier signal issynchronously demodulated and downsampled.